AlGaN/GaN field effect transistors with C-doped GaN buffer layer as an electrical isolation template grown by molecular beam epitaxy

The effectiveness of Ammonia Molecular Beam Epitaxy (MBE) grown carbon-doped GaN buffer layer as an electrical isolation template was investigated. AlGaN/GaN field effect transistor structures with a product of sheet electron density and mobility (nsμ), linearly increasing from 1.5 × 1016 V−1 s−1 to 2 × 1016 V−1 s−1 with ns, were grown on 2-μm-thick carbon-doped GaN buffer layer over sapphire substrates. The measurement of the gate-to-source voltage (VGS) dependent drain current (ID) demonstrated excellent dc pinch-off characteristics as revealed by an on-to-off ratio of 107 for a drain-source voltage (VDS) up to 15 V. The gate leakage current was less than 1 μA/mm at the subthreshold voltage (Vth = −5.2 V). Inter-devices isolation current (IISO) measurements demonstrated IISO values in the low pico-amperes ranges indicating a complete suppression of the parallel conduction paths. Small-signal rf measurements demonstrated a fmax/ft ratio as high as 2.9 attesting the absence of charge coupling effects.